The picture quality of a projection type television set has been improved remarkably as compared with that several years ago and is now approaching that of a direct viewing type. Particularly, the development of a projection lens as a key device which determines the quality of picture is outstanding. Under such circumstances we have developed a set using a projection lens of a short projection distance capable of attaining both improvement in the quality of picture and a compact construction of the set. As U.S. Patent Applications relating to this type of projection lenses there are Series No. 715,950 (filed Mar. 25, 1985), No. 911,070 (filed Sept. 24, 1986) and No. 948,251 (filed Dec. 31, 1986).
The projection lenses disclosed in the above applications are superior in focus performance and compactness.
Also as to contrast performance, improvement has been made, as disclosed in U.S. Ser. No. 911,070, by filling the space between the surface of a cathode-ray tube and a concave lens positioned in front thereof with a medium having a refractive index close to that of the concave lens and the CRT glass to reduce reflection at the surface of the concave lens on the side of the cathode-ray tube.
However, in the working examples of the said U.S. Ser. No. 911,070 no consideration is given to reflected light at the air-side surface of the concave lens and at other lens interfaces, so the reflected light at a lens interface returning to the phosphor surface is not fully removed. To avoid this inconvenience, in the projection lens disclosed in U.S. Ser. No. 948,251, the shape of the phosphor surface is changed to make the occurrence of reflected light difficult. But a still further means is desired to attain a higher contrast performance.
Moreover, a system structure which permits easy mounting of plural lens elements is desired in order to realize a less expensive projection lens system.